Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE), or their combination are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non-removed materials, oxygen, moisture, and other chemicals laterally, causing sidewall damage and lowering device performance.
To solve this issue, pure physical etching techniques such as ion beam etching (IBE) have been applied to etch the MTJ stack. However, due to the non-volatile nature, IBE etched conductive materials in the MTJ and the bottom electrode can be re-deposited into the tunnel barrier, resulting in shorted devices. Two approaches are usually applied to solve this issue. The first is to use high angle IBE to trim the MTJ sidewall to remove any re-deposition. The other solution is to greatly etch the MTJ so that the re-deposition can be confined below the tunnel barrier, without creating a shorting path. Both of these approaches require the MTJ hard mask to be very thick because this physical type of etch is not selective. However, this brings challenges for photolithography since a thick hard mask would require very thick photoresist, the patterns of which can easily collapse, especially when the size goes down to sub 60 nm. A new approach to enhance the IBE selectivity is needed to fully utilize the benefit of this technique.
Several patents teach methods of MTJ etching, including U.S. Pat. No. 8,324,698 (Zhong et al) and U.S. Pat. No. 8,008,095 (Assefa et al) and U.S. Patent Application 2018/0040668 (Park et al). These references are different from the present disclosure.